1. Field of the Invention
The present invention relates to a spread spectrum communication system and, more particularly, to a receiver.
2. Related Background Art
Different from the narrow band communication which is conventionally widely used, the spread spectrum communication is the system for spreading the energy of information signals to a very wide frequency band. Therefore, this communication has various advantageous features which cannot be obtained in the conventional communication and can be applied to various wide fields such as space communication, ground communication (in particular, mobile transceiver), distance measurement, instrumentation, and the like.
The spread spectrum system includes the following systems.
(1) DS (Direct Sequence) system PA1 (2) FH (Frequency Hopping) system PA1 (3) TH (Time Hopping) system PA1 (4) Pulse coding FM system PA1 (5) Hybrid system
In general, at present, the DS and FH systems are used, the TH system and pulse coding FM system are applied to the limited fields, and the hybrid system is being theoretically studied. The principle of the DS system will now be described. On the transmission side, the information signal is subjected to an ordinary modulation (primary modulation). The primary-modulated signal is then modulated by the spread pseudo noise code (spread PN code) of a wide band and transmitted as a wide band signal having a very small power density. This operation is called a spread modulation. On the reception side, the correlation with the received or incoming signal is derived by use of the same demodulating PN code as that on the transmission side. After the correlation was obtained, only the signal to be received is converted into the original primary-modulated signal of the narrow band. The other signals and interference signal become the wide band noises having a small power density. Only a desired signal is extracted by a filter. The primary modulation can use the analog system such as FM and the digital system such as PSK. In general, the PSK system by the pseudo noise (PN) code is used as the spread modulation. The ratio of the band width between the primary-modulated signal and the signal after it was spread is called a process gain. As the process gain is large, the advantages of the spread spectrum system are obtained. In general, the process gain is set to 1000 to 10000.
It is required that the demodulating PN code which is generated on the reception side has the same bit constitution and the same phase as those of the PN code in the incoming or received spread spectrum signal. Therefore, the initial synchronization (synchronization trapping) is performed to make the phase of the PN code on the reception side coincide with the phase of the PN code in the incoming signal. Next, in order to keep the phase-coincident PN code on the reception side, the synchronization holding process is performed by a delay-locked loop circuit (DLL). One frame or one burst of the spread spectrum signal consists of preamble data (consisting of, e.g., 256 bits) which is used for the initial synchronization and message data subsequent thereto.
The conventional PN code synchronization system uses the sliding correlation system in which, for example, the correlation with the reference PN code which was generated at a bit clock rate different from that of the incoming PN code is detected and the spread PN code generator is synchronized. According to the conventional sliding correlation system, the time until the synchronization is detected is so long to be, e.g., 1 to 20 seconds. This time is too long for the burst communication of tens of milliseconds which will be further developed in the future, so that such a sliding correlation system has the problems such that it is unfitted to and cannot be used for the burst communication. It is also possible to consider the constitution using an elastic surface wave convolver device (hereinafter, referred to as an SAW convolver in this specification) having the same processing time as the length of preamble data in order to reduce the synchronization detection time. However, there is the problem such that if the processing time in the SAW convolver exceeds a predetermined value, the SAW convolver enlarges in size and the mass production is difficult and the SAW convolver becomes expensive. On the contrary, a method whereby the length of preamble data is made coincident with the processing time of the SAW convolver can be also considered. However, in such a case, there is also the problem such that one period of the PN code is reduced, resulting in lack of secrecy of communication.
To control the synchronization of the PN code generator of the DLL and the like, it is important to detect the lock and unlock of the DLL unit which keeps the synchronization of the spread spectrum communication system. In the conventional techniques, in the case of detecting the lock and unlock of the DLL by the detection output after the reversal spreading, a dedicated band pass filter (BPF) of a narrow band and an envelope detector are provided and a lock/unlock signal is derived from the output of the envelope detector, or the lock and unlock are detected from the output of the envelope detector constituting the DLL.
According to the former conventional technique, there is the problem such that in addition to the band pass filter constituting the DLL and the envelope detector, it is necessary to additionally provide a band pass filter of a narrow band and an envelope detector to detect the lock and unlock.
According to the latter conventional technique, since only the output of one envelope detector, namely, only one correlation detection output is taken out, there is the problem such that the lock and unlock are detected irrespective of the output of the other envelope detector, namely, the other correlation detection output, and the lock state is detected before the stable lock state is obtained, so that the detection is uncertain.
In the receiver of the spread spectrum system, hitherto, the correlation between the output of the PN code generator for the initial synchronization (hereinafter, referred to as the reference PN code generator) and the preamble data in the incoming spread spectrum signal is detected by using two different nth-order PN code generators for the initial synchronization and for the spread spectrum demodulation, the initial synchronization is performed, and the PN code which was phase synchronized with the incoming spread spectrum signal is output from demodulation PN code generator (hereinafter, referred to as the demodulation PN code generator) for the data demodulation after the initial synchronization, thereby keeping the synchronization and demodulating the data.
According to the conventional technique mentioned above, the reference PN code generator and demodulation PN code generator are respectively independently constituted.
Therefore, there is the problem such that the shift register constituting a part of the reference PN code generator and the shift register constituting a part of the demodulation PN code generator are independent.